Wait, I can't just say "It's common knowledge"?
Anyway, no I'm not basing that off of what "fringe TRT clinics" say. There's actually quite a bit of literature on this, particularly recently (and I'm not just talking about all of the recent literature studying it for replacement therapy):
https://www.mdpi.com/1999-4923/12/6/56
Systemic absorption of a drug depends on its physicochemical properties, the nature of the dosage form on which it is included and the anatomical and physiological characteristics of the site of absorption. These considerations are important on the biopharmaceutical...
link.springer.com
Per your assertion that "subq is the opposite of that", that's contradicted in virtually all literature I've seen,
As the blood perfusion in the s.c. tissue is lower than in the muscles, a s.c. injection will result in a delayed absorption of the drug with respect to an i.m. injection [17].
Interestingly, while the consensus in this thread is that SubQ is only viable for "TRT amounts", there are numerous references in literature about much larger volumes of oil-based compounds:
S.c. injections are the most-often used parenteral administration route due the possibility of self-injection by the patients, but the administered volume is limited. The administration of volumes larger than 1.5 to 2 mL usually requires multiple injections to minimize the risk of considerably high pain upon injection.
I've never gone higher, but I've personally had no issues running up to 1.1ml per injection.
wrt my assertion that oil-based on compounds weren't necessarily designed for IM per se, when Test Prop was introduced it was used exclusively IM, but with the introduction of Test Cyp and Test E in the late 1950s.
Primary sources for this are buried in very old literature I haven't seen in ages, but plenty of references exist for this in recent literature.
The biological effects of the testes and testosterone are known since antiquity. Aristotle knew the effects of castration and his hypothesis on fertilization is one of the first scientific encounters in reproductive biology. Over centuries, ...
www.ncbi.nlm.nih.gov
Injections with intramuscular (IM) testosterone esters have been available for almost 8 decades and not only result in predictable serum testosterone levels but are also the most inexpensive modality. However, they are difficult to self-administer ...
www.ncbi.nlm.nih.gov
That last study has a pretty neat infographic showing the timeline of various testosterone formulations from 1889 to present.
Interestingly, while Test Prop (and later) Test Undecanoate were designed for IM injection, Test Cyp and Test E were used both IM and SubQ from the beginning.
IM was certainly more popular in the beginning, but the decision was never based on "these are designed for IM and aren't designed for SubQ".
Regardless, the is study after study coming out proving the viability of SubQ injections, I'm not going to dig up/post all of the links but from the references at the above links you could pull a dozen+ easily.
FWIW, there's some evidence that SubQ may be preferable for athletes, because the distribution of depot-based injections is heavily influenced by lymphatic drainage. In SubQ tissue, this is relatively stable, but is increased by physical activity IM:
Therefore, the pharmacokinetics of testosterone esters administered via IM vs SC route will vary according to the lymphatic circulation of the tissue. Lymphatic drainage is dependent on intrinsic and extrinsic pumping. Intrinsic pumping is dependent on the contraction of lymphangions (muscular unit of the lymphatics with unidirectional valves) that transport lymph by mechanisms analogous to that occurring in the cardiac chambers (42). Extrinsic pumping results from intermittent external pressure exerted by skeletal muscle contractions on the lymphatics (42). As the lymphatic drainage from SC tissue is largely dependent on intrinsic pumping, while IM lymphatic flow is also substantially influenced by extrinsic pumping during physical activity (43), these drainage patterns suggest that testosterone esters administered SC likely have more stable absorption kinetics compared to IM administration.
IMO the science is quite clear that SubQ injections are very viable, but for whatever reason bodybuilders are some of the worst I've seen about getting irrationally angry at anything evolving or changing. This is definitely one of those situations.